The present invention relates generally to giant magnetoresistive (GMR) transducers for retrieval of data in magnetic recording disc drives. In particular, the present invention relates to a spin valve type head for perpendicular recording.
A magnetoresistive read head retrieves magnetically-encoded information that is stored on a magnetic medium rigid disc. The magnetoresistive read head is typically formed of several layers that include a top shield, a bottom shield, and a read-element positioned between the top and bottom shields.
The read-element is generally a magnetoresistive thin film that exhibits spin-dependent scattering giant magnetoresistance effect. The resistance of a read-element changes in response to a magnetic field emanating from a magnetic medium when the GMR read head, flowing on a slider on a air bearing is positioned near a magnetic transition of the magnetic medium.
The magnetic fields that are produced by the magnetic patterns of the magnetic medium, corresponding to streams of xe2x80x9c1xe2x80x9d and xe2x80x9c0xe2x80x9d, are sensed by the magnetoresistive element. By applying electrical current to the GMR read-element, the change in resistance in the read-element produces a voltage signal to first order linearly proportional to the local medium magnetic flux.
A commonly used GMR transducer is the spin valve head in which the read-element consists of a multi-layered structure formed of a ferromagnetic soft layer (free layer), a ferromagnetic hard layer (reference layer) and a nonmagnetic spacer layer positioned between the free layer and the reference layer.
The magnetization direction of the hard layer is typically set normal to the air bearing surface between the read head slider and the disc surface. The resistance of the sensor can be varied from a maximum value to a minimum value, by changing the orientation of the magnetization of the free layer from antiparallel to parallel to the reference layer, respectively.
One way to prepare a hard magnetic layer that exhibits GMR effect consists in preparing a soft layer exchange coupled to an antiferromagnetic layer. An antiferromagnetic layer that is in atomic contact with a soft ferromagnetic film can induce a unidirectional anisotropy field in the soft layer, which strength depends on the interface coupling energy between the couple.
The pinning field amplitude generally depends on the interfacial atomic and magnetic properties of the soft layer and the antiferromagnetic layer and on the temperature. The hard layer unidirectional anisotropy, exchange induced by an antiferromagnet can be set during film growth or by thermal treatment in an applied field.
The increase in storage capacity and data rate in disc drives has been accomplished by decreasing the bit length and the bit aspect ratio. These typically implicate that the track-width and the gap length of the read head are decreased and that the thickness of the magnetoresistive element and of the magnetic medium layer are decreased.
The decrease in the bit volume has been predicted and observed to cause erasing of the magnetic patterns. This time and temperature dependent loss of the recording bits information may limit longitudinal recording to areal densities below 300 Gbit/in2. On the other hand, perpendicular recording media could exhibit thermally stable areal bit densities beyond 100 Gbit/in2.
As in concurrent longitudinal recording, in perpendicular recording the data is recorded with a thin film inductive head and read-back with a magnetoresistive head. As the bits are packed in smaller areas to provide larger storage capacity and higher data rates, magnetoresistive heads providing higher signal amplitude and higher linear resolution are required.
Moreover, perpendicular recording media generate asymmetric fields that are converted into voltage signals by magnetic flux detecting magnetoresistive heads. Asymmetric read-back signals may require considerably different signal processing than that being applied in longitudinal recording.
Both spin valves and AMR type sensors have been tried with perpendicular recording media. These sensors produce an output signal referred to as a bipolar pulse. A electrical circuit that uses a high pass filter and consequently transforms bipolar pulses into unipolar pulses, has been applied to process with current channels the output signals of those read heads.
For several decades, disc drives use longitudinal recording and the signal processing that converts analog signals into digital bits is adequate for unipolar pulses. The use of differentiators results in the amplification of noise at high frequencies. At high linear densities (smaller bits) and high data rates, electronic noise makes the use of differentiators impractical.
In this invention, a GMR read head provides enhanced sensitivity by using a read-element of two spin valves, operating in a current perpendicular to the plane geometry (CPP). The head is provided with high linear resolution and differential operation by incorporating an interlayer between the free layers of the two spin valves.
The differential mode of operation does not require shields for the head linear resolution and is accomplished by incorporating a thin gap layer between the free layers of the spin valves. The thickness of this interlayer can easily be made of the order of the bit length and defines the head intrinsic linear resolution.
The thin metallic gap also electrically connects the two spin valves in serial. Hence, the total signal amplitude is proportional to the sum of the variation of resistance of two spin valves. Moreover, the CPP spin-dependent scattering variation of resistance with applied field can be a factor of three higher than that commonly obtained with the current in the plane geometry.
The current perpendicular to the plane geometry is beneficial for the head sensitivity and track-resolution. The former is a consequence of the fact that in the CPP geometry, the variation of resistance increases with the decrease of the track-width. The latter results from the fact that the top and bottom electrodes do not define the magnetic read width.
The incorporation of a reproduce gap, defined by a metallic interlayer between the two magnetic flux sensor free layers is beneficial for high data rate magnetic recording disc systems. It provides the head with a differential character and a linear resolution that may be ultimately limited by the thickness of the interlayer, of the order of the bit length.
Moreover, the CPP differential dual spin valve head of in this invention is suitable for the retrieval of digital information stored in perpendicular disc media because it produces unipolar pulses similar to those obtained for decades with longitudinal recording. This property could pose less challenges to the signal processing of perpendicular recording disc drives.
The CPP differential dual spin valve (DDSV) read head of this invention includes a read-element that consists of a first and second spin valve stacks configured to operate in a current perpendicular to plane (CPP) geometry and in high linear resolution differential mode by incorporating a thin interlayer between two spin valve free layers.
The first and the second spin valve stacks each include a free layer and a metallic spacer is positioned adjacent to the free layer of the first and the second spin valve stacks and spaces the first spin valve stack apart from the second spin valve stack. By this way, no shields are required for the head linear resolution.
A magnetic permeable material can be incorporated between the write pole and the read-element multilayer to protect the read head from the write fringing fields. Nonethless, this head is configured to operate in a differential mode, in which, shields are not incorporated to define the head linear resolution.